This invention relates to novel porous separators for electric lead-acid storage batteries. According to another aspect the invention relates to lead-acid storage batteries comprising such a novel separator.
Basically, battery separators serve as electronic insulators and ionic conductors, i.e. they prevent the direct electronic contact of electrodes of opposite polarity while enabling the ionic current between them. To meet these two functions, separators are usually porous insulators with pores as small as possible to prevent electronic short circuits by dendrites or plate particles and with a porosity as high as possible to minimize the internal battery resistance. In lead-acid batteries, the separator also determines the proper plate spacing and thereby defines the amount of electrolyte which participates in the cell reaction. The separator has to be stable over the life time of the battery, i.e. to withstand the highly aggressive electrolyte and oxidative environment.
Beyond these basically passive functions, separators in lead-acid batteries can also actively affect the battery performance in many ways. In valve regulated lead-acid (VRLA) batteries they additionally determine properties like oxygen transfer, electrolyte distribution and plate expansion. Due to their outstanding influence on the performance of VRLA batteries the separator is even referred to as the xe2x80x9cthird electrodexe2x80x9d or xe2x80x9cfourth active materialxe2x80x9d (Nelson B., Batteries International, April 2000, 51-60).
VRLA stands for valve-regulated lead-acid batteries which are also called sealed or recombinant batteries. In VRLA batteries oxygen, which is generated during charging at the positive electrode, is reduced at the negative electrode. Thus the battery can be charged and even be overcharged without water consumption and is therefore theoretically maintenance-free. The formation of hydrogen at the negative electrode is suppressed, for instance by using larger negative than positive plates in order to generate oxygen at the positive plate before the negative plate is fully charged.
For VRLA batteries two technologies are predominant, i.e. batteries with an absorptive glassmat (AGM) and gel batteries. In batteries with AGM, the absorptive glassmat immobilizes the electrolyte and simultaneously functions as a separator. In gel batteries, the acid is immobilized by means of fumed silica and an additional separator is required to fix the plate distance and to prevent electronic shorts. Compared to AGM batteries, the manufacturing cost of gel batteries is considered to be higher and their specific power is lower due to a higher internal resistance.
In AGM batteries the electrolyte is completely absorbed by the glass mat. AGM separators have a very high porosity in excess of 90%. This high porosity together with a good wettability is reflected in a very high acid absorption and low electrical resistance. In the battery, the acid saturation of AGM separators is usually in a range of 85 to 95%. This increases the effective electrical resistance versus fully saturated separators but creates open channels of relatively large pores that enable a very efficient oxygen transfer from the positive to the negative plate. The average pore size of AGM separators is usually within the range of 3 to 15 xcexcm with an anisotropic distribution, i.e. pore sizes of about 0.5 to 5 xcexcm in the x-y-plane of the separator which is the plane parallel to the electrode plates and pore sizes of about 10 to 25 xcexcm in the z-direction perpendicular to the electrodes. A potential drawback of the high oxygen transfer rate is the so-called thermal runaway, caused by the self-propelling exothermic consumption of oxygen at the negative plate and a premature capacity loss by undercharging of the negative plate.
Due to the relatively large pores and the good wettability, the wicking rate (speed of acid pick-up) of AGM is fairly high which facilitates the filling process of batteries.
A severe disadvantage of AGM separators is their mechanical weakness which is due to the fact that pure glass separators do not contain binders of any type. The tensile strength of this material depends only on the fiber contacts and some entanglement. At the molecular level these contacts are believed to be of the hydrogen bonding type established between adjacent fibers. Since finer fibers have greater chances to establish these contacts, it follows that the strength of the material is greatly influenced by their presence.
On the other hand coarser glass fibers also play a role in the ability of the AGM separators to serve its many functions. For instance, they improve the wicking rate by creating larger pores.
In an approach to benefit from both the advantages of fine and coarse glass fibers, multi-layered AGM separators have been proposed. It could be shown that two layers with fine and coarse fibers showed e.g. a better tensile strength as if these fibers would have been dispersed in one sheet (Ferreira A. L.; The Multilayered Approach for AGM Separators; 6th ELBC, Prague Czech Republic, September 1998).
U.S. Pat. No. 5,962,161 discloses separators made from a mat of meltblown ultrafine polymer fibers which may be reinforced with one ore more thin layers of spunbond fabric.
U.S. Pat. No. 4,908,282 discloses fibrous sheet separators comprising a mixture of glass fibers and polyethylene fibers.
It also has been suggested to include thin microporous sheets as part of the separator system in order to control dendrite formation and oxygen transfer to the negative plate. An example of such a microporous separator is the DuraGard(trademark) separator introduced by ENTEX International LLC (Weighall M. J.; ALABC Project R/Sxe2x80x94001, October 2000). DuraGard(trademark) has an average pore size of 0.014 xcexcm and a membrane thickness of 0.10 mm (Fraser-Bell G., New developments in Polyethylene separators, Presentation at the 7th European Lead Battery Conference, Sep. 19-22, 2000, Dublin, Ireland).
However, if the separator has a very small pore size, it will act as a barrier to oxygen transport, and the gas will rise to the top of the plates and go over the top or around the sides of the barrier layer of the separator. This means that only the top and edges of the negative plate will participate in the oxygen reduction reaction. This is not a desirable situation as the oxidation of the pure lead in the negative plates is a highly exothermic reaction, resulting in a build up of heat in a very localised area. This results in increased danger of premature water loss and deactivation of the negative plates. It was therefore suggested to use a separator with a larger average pore size, for example a microporous PVC separator having a mean pore size of 5 xcexcm and a thickness of 0.57 mm, sandwiched between two layers of AGM with a thickness of 0.52 mm at 10 kPa (Weighall M. J., see above, Lambert U., A study of the effects of compressive forces applied onto the plate stack on cyclability of AGM VRLA batteries, 5th ALABC Members and Contractors"" Conference Proceedings, Nice, France, Mar. 28-31, 2000). Although this separator configuration might provide for acceptable oxygen transfer and improved resistance to dendrite growth when compared to AGM separators, the risk of shorting is still existing. Moreover, due to the outer AGM layers these separators are difficult to form into pockets.
The present invention relates to a battery separator for a lead-acid battery comprising at least one microporous polymer layer and at least one fibrous layer, wherein said microporous polymer layer comprises micropores with an average pore size of less than 1 xcexcm and a number of holes with a diameter which is greater than the average diameter of the pores of the fibrous layer.
It is the object of the invention to provide a battery separator for a lead-acid battery with improved tensile strength without impairing the oxygen transfer.
It is a further object of the invention to provide a battery separator which can be produced in a cost effective manner.
It is still a further object of the invention to provide an improved valve-regulated lead-acid battery with high cycling performance.